Process and installation for recovery and purification of ethylene produced by pyrolysis of hydrocarbons, and gases obtained by this process

ABSTRACT

A process for the high-yield recovery of ethylene and heavier hydrocarbons from the gas produced by pyrolysis of hydrocarbons in which the liquid products resulting from the fractionated condensation of a cracking gas for the recovery of almost all the ethylene, are supplied to a distillation column, called a de-ethanizer, at different intermediate levels. At the top of the de-ethanizer the vapor of the column distillate is treated directly in an acetylene hydrogenation reactor, the effluent containing virtually no acetylene being separated by a distillation column called a de-methanizer, into an ethylene- and ethane-enriched tail product, while the head product is recycled by compression or treated for subsequent recovery of ethylene.

FIELD OF THE INVENTION

This invention concerns in general and in a first of its aspects, thechemical industry and, in particular, a method for high-yield recoveryand purification of ethylene as well as other products originating froma gas produced by pyrolysis of hydrocarbons. This invention alsoconcerns an installation and equipment for exploiting this method on anindustrial scale.

BACKGROUND

A large number of papers and patents addressing the production,recovery, and purification of olefins show their industrial importanceand the problems encountered in the exploitation of the variousprocesses.

Recently, the production capacity of ethylene units has attained andeven exceeded the level of 1 million tons per year for a single line;which requires a new approach in the design of the process, equipment,and the controllability of the unit.

In systems of recovery and purification, particularly for ethylene, theelimination of acetylene is a key element in purification. Because ofits relative volatility with respect to ethylene and ethane, it cannotbe separated by distillation. In industrial practice, only two processesare applied: absorption of acetylene by a solvent and hydrogenation toethylene and ethane.

The first method involves the use of a solvent which is usuallyN,N-dimethylformamide (DMF) or N-methylpyrrolidone (NMP), which allowsfor preferential recovery of dissolved acetylene.

The second method, catalytic hydrogenation, is generally carried out bytreatment of all the gas from cracking before separation of the hydrogencontained in it, or a separate treatment of the cuts containing C₂hydrocarbons after addition of sufficiently pure hydrogen to transformall the acetylene into ethylene and ethane. These two types ofhydrogenation use palladium-based catalysts with different formulations.

The stage of hydrogenation of acetylene has also been the subject of anumber of papers and inventions dealing with the catalyst system and theformulations of the catalyst, and exposing the specific disadvantagesconnected with each of the hydrogenation technologies.

Thus, in the case of treatment of all the cracking gas originating fromthe pyrolysis of hydrocarbons in a hydrogenation reactor, a racingreaction may occur, corresponding to an acceleration of the kinetics ofthe reaction transforming the acetylene into ethylene (and alsoundesirable secondary reactions) because of a significant increase inthe temperature of the catalyst together with the presence of a largeexcess of hydrogen (50 to 100 times the quantity required bystoichiometry). The ethylene can then be transformed into ethane and maythereby cause a significant rise in temperature, which requiresimmediate depressurizing of the reactor to prevent an explosion.

In the case of treatment of the C₂ cut alone, polymerization of theacetylene and progressive deactivation of the catalyst may occur,because of the large concentration of unsaturated hydrocarbons in thecut to be treated, which necessitates regeneration or periodicreplacement of the catalyst charge. Generally, a reserve reactor isinstalled to avoid interrupting production. In addition, it is necessaryto use a purified hydrogen current for the reaction, and these twoaspects tend to increase the investments for reserve equipment or theequipment used only for the purpose described.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the known previoustechniques by purification of the ethylene-rich fraction at anintermediate stage of the process.

Thus the invention concerns, according to one of its aspects, a processfor fractionation of a large anhydrous gas resulting from pyrolysis ofhydrocarbons containing hydrogen and hydrocarbons, particularly C₁ to C₃hydrocarbons, including ethylene, propylene, and acetylene, and at leastone current enriched with hydrogen and/or methane, at least one currentenriched with ethylene and poor in acetylene, and at least onepropylene-rich current, including stages wherein:

-   -   a) the gas resulting from the pyrolysis of hydrocarbons under        pressure is cooled and liquefied progressively by passage into a        series of increasingly colder heat-exchange zones. At least one        condensate is separated from the pyrolysis gas after passage        into each heat-exchange zone, at least one of the condensates        being propylene-enriched and at least one other condensate being        ethylene and ethane enriched, and containing in solution a        smaller proportion of hydrogen, methane, and acetylene, and the        residual hydrogen-rich gas is collected;    -   b) at least part of the ethylene- and ethane-enriched condensate        and the propylene-enriched condensate is evaporated by a        decrease in pressure. They are reheated, independently or not,        in at least one of the heat-exchange zones by thermal exchange        with fluids to be cooled, including at least the gas resulting        from the pyrolysis, to provide, respectively, a fraction that is        at least partly evaporated due to the reduction in pressure and        reheating of the ethylene- and ethane-enriched fraction, and a        fraction that is at least partly evaporated due to the reduction        in pressure and the reheating of the propylene-enriched        fraction, to provide at least part of the cold needed for        cooling and for progressive liquefaction of at least said gas        resulting from the pyrolysis of hydrocarbons upon passage into        said successive heat-exchange zones;    -   c) the fractions which are at least partly evaporated, resulting        from stage (b), are introduced into part of a distillation        column called a de-ethanizer, the ethane- and ethylene-rich        partly evaporated condensate being admitted into a point of the        part of the distillation column called the de-ethanizer, higher        than the propylene-enriched partly evaporated condensate, the        part of the distillation column called the de-ethanizer        operating under conditions of temperature and pressure allowing        the separation, in an upper part, of a first current of        ethylene- and ethane-enriched head gas containing, in a smaller        proportion, acetylene, hydrogen, and methane, and in a lower        part, a first bottom current of propylene-enriched fluid, which        is collected;    -   d) the first current of ethylene- and ethane-enriched head gas        from stage (c) in a zone of acetylene elimination by extraction        with solvent and/or by selective hydrogenation of the acetylene        by means of hydrogen containing in the first gaseous head        current, to provide a current essentially devoid of acetylene,        and    -   e) in the part of the distillation column called the        de-methanizer, the gas current which is essentially devoid of        acetylene from stage (d) is cooled and fractionated in a second        hydrogen- and/or methane-enriched head gas fraction, which is        collected, and a second bottom liquid fraction which is enriched        with ethylene and ethane, and is essentially devoid of        acetylene, and which is also collected.

The charge gas is generally essentially free of water to preventdeposits of ice in the low-temperature circuits. Thus, a water contentlower than 10 ppm by volume, preferably less than 1 ppm, is desirable.

According to one of its aspects, the process according to the inventionmay use the gas current from the pyrolysis of hydrocarbons at a pressureof 15-50 bar, preferably 28-38 bar, and the distillation zone called thede-ethanizer may be at a pressure of 10-30 bar, preferably 14-24 bar,lower than the pressure of the pyrolysis gas.

According to one of its aspects, the process according to the inventionmay use evaporated condensates introduced into the part of thedistillation column called the de-ethanizer; these condensates containdissolved hydrogen in a proportion such that the first head gas currentcontains 2 to 10%, preferably 4 to 5%, in moles, of hydrogen, and stage(d) may be implemented by essentially ethylene-selective hydrogenationof the acetylene contained in the first head gas current by means of thehydrogen contained in the first head gas current of stage (c), thetemperature of the hydrogenation zone being between 0 and 160° C.,inclusive.

According to one of its aspects, the process according to the inventionmay use the hydrogen dissolved in the evaporated condensates introducedinto the part of the distillation column called the de-ethanizer, sothat it is the only hydrogen used for the hydrogenation carried out instage (d).

according to one of its aspects, the process according to the inventionmay be implemented by sending into the top of the de-ethanizer of stage(c) two or three condensates obtained after successive passage of thepyrolysis gas, respectively, into two or three last heat-exchange zonesof stage (a), considering that the first heat-exchange zone is the onewhich is first to be in contact with the pyrolysis gas.

The pyrolysis gas may be, for example, a naphtha or ethane pyrolysisgas.

According to one of the aspects of the process according to theinvention, the second head gas fraction from the de-methanizer may bepurified by distillation to recover ethylene and ethane.

According to one of the aspects of the process according to theinvention, the pyrolysis gas may be an ethane pyrolysis gas, or a gasfor pyrolysis of the ethane/propane mixture, and the second head gasfraction from the de-methanizer may be mixed with the pyrolysis gaswithout ethylene recovery, for a new treatment in mixture with thepyrolysis gas in stage (a).

According to one of the aspects of the process according to theinvention, the hydrogen content of the first head gas current from thede-ethanizer may be increased by the addition of hydrogen from the headof a fluid separator, this fluid coming from the cooling in aheat-exchange zone of the residual gaseous fluid resulting fromrefrigeration in the successive heat-exchange zones of the pyrolysisgas.

According to one of the aspects of the process according to theinvention, part of the second bottom liquid fraction from thede-methanizer is recycled into the de-ethanizer, to reduce the acetyleneconcentration of the first head gas current from the de-ethanizer.

According to one of the aspects of the process according to theinvention, stage (d) can be carried out by extraction of the acetyleneby means of a solvent.

According to one of the aspects of the process according to theinvention, the carbon monoxide concentration contained in the first headgas current may have a moderating effect on the catalyzed reaction ratein the acetylene elimination zone.

According to another of its aspects, this invention concerns aninstallation for the fractionation of a gas resulting from pyrolysis ofhydrocarbons containing hydrogen and hydrocarbons, particularly C₁ to C₃hydrocarbons, including ethylene, propylene, and acetylene, and at leastone current enriched with hydrogen and/or methane, at least one currentenriched with ethylene and poor in acetylene, and at least one currentenriched with propylene, comprising:

-   -   a) means for progressively cooling and liquefying the gas from        the pyrolysis of hydrocarbons under pressure by passage into a        series of increasingly colder heat-exchange zones, means for        separating from the pyrolysis gas at least one condensate after        passage into each heat-exchange zone, at least one of the        condensates being propylene-rich and at least one other        condensate being ethylene- and ethane-rich, and containing in        solution a smaller proportion of hydrogen, methane, and        acetylene, and means for collecting the hydrogen-rich        uncondensed residual gas;    -   b) means to evaporate, at least in part, by reduction in        pressure, the ethylene and ethane-rich condensate and the        propylene-rich condensate, and means to heat them independently        in at least one of the heat-exchange zones by thermal exchange        with the fluids to be cooled, to provide, respectively, a        fraction that is at least partly evaporated due to the reduction        in pressure and the heating of the ethylene- and ethane-rich        fraction, and a fraction at least partly evaporated due to the        reduction in pressure and the heating of the propylene-rich        fraction, to provide at least part of the cold necessary for        progressive cooling and liquefaction of at least the gas        resulting from the pyrolysis of hydrocarbons upon its successive        passage into the heat-exchange zones,    -   (c) means to introduce the partly evaporated fractions resulting        from stage (b) into part of a distillation column called the        de-ethanizer, the partially evaporated ethylene- and ethane-rich        condensate being admitted at a point of the part of the        distillation column higher than the partly evaporated        propylene-rich condensate, the part of the distillation column        operating under conditions of temperature and pressure        permitting the separation, in an upper part, of a first        ethylene- and ethane-rich head gas current containing, in a        lower proportion, acetylene, hydrogen, and methane, and in a        lower part, a first propylene-rich liquid bottom current, which        is collected,    -   d) means to send the first ethylene- and ethane-rich head gas        current resulting from stage (c) into an zone for acetylene        elimination by extraction with a solvent and/or by selective        hydrogenation of acetylene by means of the hydrogen contained in        the first head gas current, to provide an essentially        acetylene-free current, and    -   e) means of cooling and fractionation, in part of a distillation        column called the de-methanizer, of the essentially        acetylene-free gas current from stage (d) and a second head gas        fraction, rich in hydrogen and/or methane, which is collected,        and a second ethylene- and ethane-rich bottom liquid fraction,        essentially free of acetylene, which is also collected.

BRIEF DESCRIPTION OF DRAWING FIGURES

The invention is described with reference to the attached diagrams, andby showing two modes of embodiment of the invention as nonlimitingillustrations.

In the diagrams:

FIG. 1 illustrates schematically the treatment of a gas resulting frompyrolysis of the ethane/propane and/or the liquefied petroleum gas(LPG);

FIG. 2 shows the same treatment applied to the gas resulting frompyrolysis of naphtha and heavier hydrocarbons.

For FIG. 1, the number 1 designates the feed line of the sufficientlydry pyrolysis gas (for example, 10 ppm water) at a pressure that isselected based on the required hydrogen pressure (for example, 15 to 50bar, preferably between 28 and 38 bar).

The typical composition of a gas obtained by ethane cracking is shown inthe table below (in mol %):

Cut Cut CO/ H₂ CH₄ C₂H₂ C₂H₄ C₂H₆ C₃ C₄ C₅ ₊ CO₂ TOTAL 36.9 5.4 0.3 34.221.0 0.6 0.6 1.0 0.6 100.00

This gas is cooled in the multiple-flow beat exchanger 2 and inexchanger 3, respectively, by heat exchange with the cold gases producedby vaporization of the condensates mentioned below and by vaporizationof propylene circulating in the traditional cooling loop in a closedcircuit.

The partly condensed gas is supplied to separator 4 at a temperaturepreferably between −30 and −40° C.

The gas 5 collected at the head of the separator 4 is further cooled, inthe multiple-flow heat exchanger 6, by the cold gases produced byvaporization of the condensates mentioned below, and additionally byproduct 43, which is the C₂ cut from the bottom of the de-methanizer C2.

The partly condensed gas in the exchanger 6 is then is supplied to theseparator 7 at a temperature preferably between −45 and −55° C. The gas8 collected at the head of the separator 7 is further cooled in the heatexchangers 9 and 10, respectively, by the cold gases, as above, and byvaporization of ethylene circulating in a closed-circuit cooling cycle.

The partly condensed gas is supplied to the separator 11 at atemperature preferably between −65 and −75° C.

The liquid exiting the separator 11 is preferably divided into twoparts, product 12 and product 13, previously reduced in pressurerespectively, in gates 12A and 13A.

The product 12 flows in countercurrent to the feed gas through the heatexchangers 9, 6, and 2, and is supplied to the top section of thedistillation column C1. Product 12 is partly vaporized before being fedinto column C1, called the de-ethanizer, because its role is essentiallyto separate ethane, ethylene, and the lighter top hydrocarbons, andpropylene and the heavier fractions at the bottom of the column.

The liquid product 14 from the separator 7 is reduced in pressure in thegate 14A, and flows in countercurrent to the feed gas in the heatexchanger 6, mixes with the pressure-reduced product 15 in gate 15A fromthe bottom of the separator 4, then flows in countercurrent to thepyrolysis gas into the heat exchanger 2. It is then vaporized bycirculating in the multiple-flow exchanger 16. The product heating saidexchanger may be propylene from the closed circuit cycle or any otherappropriate hot fluid. The product exiting 17 essentially contains theC₃ hydrocarbons and the heavier hydrocarbons contained in the feed gas1, with the exception of those contained in products 12 and 13, and issupplied to column C1 at the level of its middle section. In a variant,products 14 and 15 are sent separately into column C1.

The distillation column C1 is heated by a hot fluid in a heat exchangerREB1, which is, for example, either hot water from the process orlow-pressure vapor depending on the temperature, preferably between 60and 80° C., connected to the bottom composition of the column. Theservice pressure of the column C1 is preferably between 14 and 24 bar.

The product 18 drawn off the bottom of the column C1 can be treated inthe traditional manner to recover the propylene in a unit, not shown,located downstream from the process.

The gas current 19 from the separator 11 is further cooled in a heatexchanger 20 by low-temperature gases as described below, and in theheat exchanger 21 by vaporization of ethylene from a closed-circuitcooling cycle. The partly condensed gas 22B obtained at a temperaturebetween −90 and −100° C. is supplied to the ethylene separator 22 at thebottom part 22C.

The liquid product 23 collected at the foot of the ethylene separator 22is reduced in pressure in gate 23A, heated by the feed gas in the heatexchanger 20, possibly mixed with product 13 exiting separator 11 andwhich has been reduced in pressure in gate 13A, and is heated again andpartly vaporized in the heat exchangers 9 and 6. The resulting product25 is the reflux from column C1.

The process and-the equipment described represent the first remarkablecharacteristics of the invention, in which:

The column C1, which is a de-ethanizer, does not need a top condensationsystem and the associated equipment.

The head product of column C1 is a C₂ cut which contains a certainquantity of methane and hydrogen dissolved in the condensates from theseparators 4, 7, 11 and 22. This quantity is lower than in theequivalent columns of the previously known processes, and offers anadditional cost advantage.

The pressure of column C1 may be chosen in a range allowing for a lowbottom temperature and making it possible to avoid the known phenomenaof temperature-related clogging.

Returning now to separator 22, the gas 24 from the head of the lowerpart 22C of the separator 22 is further cooled in the heat exchanger 26to a temperature generally lower than −120° C.

The gas 24, cooled and partly condensed, is reintroduced as product 24Cinto the separator 22 in its upper part 22D, above the lower part 22C.The condensed fraction separated in the upper part 22D of the separator22 is introduced into a series of pipes 24A equipped with a hydraulicguard, and is then introduced into the top of the lower part 22C.

The gas fraction 27 exiting the top of the upper part 22D of theseparator 22 is composed of a mixture of hydrogen, methane, carbonmonoxide, and traces of ethylene. The product 27 is heated in the heatexchangers 26 and 20 before it is reduced in pressure in the turbine 28.

The product 30 exiting the turbine 28 is heated by the feed gas in thecomplete series of exchangers 26, 20, 9, 6, 2, and 16, before it iscompressed in the machine 29, which is harnessed to turbine 28. Theproduct 30A is discharged from the process.

The head distillate of column C1, the product 31, is heated in thecharge/effluent heat exchanger 32 and in the heater 33 before enteringthe catalytic reactor R1. The role of this reactor is selectivehydrogenation of the small quantity of acetylene, generally less than 1%in moles, and transformation of it into ethylene and ethane. Thiscatalyst system is based on a known type, for example, palladium-based,and does not require additional description. The temperature is, forexample 0 to 160° C.

The second notable characteristic of the invention, compared to a knownequivalent technique, is the fact that hydrogenation is carried out on amixture of gas that already contains sufficient hydrogen in addition tothe 3 hydrocarbon components of the C₂ cut, ethylene, ethane, andacetylene, to accomplish the reaction under moderate and safeconditions. Pure hydrogen need not be added.

The typical composition of product 31 is shown in the following table:

H₂ CO CH₄ C₂H₄ C₂H₆ C₂H₂ C₃ ₊ TOTAL % mol. 4.70 0.04 4.86 57.10 32.400.80 0.10 100.00

The advantages over the prior art are as follows:

A lower total volume flow, hence a reduced catalyst volume, because ofthe presence of a small proportion of hydrogen, for example 4 to 5% inmoles of hydrogen (more generally 2 to 10%), compared to 30-40% in thetraditional systems.

Safer operation because of the decreased risk of a runaway reaction inthe case of uncontrolled exothermic hydrogenation of ethylene.

Addition of purified hydrogen is not needed to supply the reactor.

Catalyst poisons are partly eliminated by condensation and fractionationof the C₂ cut in the de-ethanizer. The small quantity of carbon monoxidepresent in the mixture to be hydrogenated has a beneficial moderatingeffect on the conduct of the hydrogenation, because this makes itpossible to limit the frequency and the rapidity of any runawayreactions that may occur.

The effluent from reactor R1 containing virtually no acetylene is cooledin the heat exchanger 34, then passes through the charge/effluent heatexchanger 32 and the protection dryer 35 containing a dehydrating agent,for example a molecular sieve (zeolite) or similar, to yield a dried gas36.

The product 36 containing residual hydrogen and methane, in addition tothe ethylene and ethane, is cooled in the reboiler REB2 and the columnC2 and the subcooler 37 using propylene resulting from a closed-circuitcooling cycle. The partly condensed product 38 is supplied to theseparator 39. The gas 40 collected at the head of the separator 39 isthen cooled in the heat exchangers 6 and 41, respectively, with coldgases in the multiple flow exchanger 6 and with vaporized ethylene froma closed circuit cooling cycle in the cooler 41, and is supplied to thetop of the column C2. The liquid 42 coming from the bottom of theseparator 39 is supplied to the middle zone of the column C2. The columnC2, called the de-methanizer, operates at a pressure of 10 to 16 bar,separates the components lighter than ethylene at the top, and leavesthe purified C₂ hydrocarbons in the bottom product 43.

The distillate 44 exiting the head of the column C2 is sent to the heatexchangers 9, 6, 2, and 16 to be heated, and it can finally be recycledto the gas compression system outside the field of application of thisinvention.

The ethylene-rich bottom product 43 undergoes pressure reduction in gate43A; it is heated and partly vaporized in the exchanger 6, and can besupplied to a traditional ethylene purification column, not shown.

According to this description, the preferred technology for eliminatingacetylene is the hydrogenation performed on the product 31, because theacetylene is transformed into ethylene and ethane, which are productswith more economic potential. However, if we wish to preserve theacetylene, a solvent extraction system can be applied to product 31,which would replace the whole circuit of the hydrogenation reactor, withequipment R1, 32, 33, 34, and 35. This constitutes another advantageover the processes using hydrogenation of acetylene over the entirecracking gas, which requires modification to adapt it to a solventextraction system.

For FIG. 2, the number 1 designates the feed line of the cracking gashaving a typical composition indicated below, in mol %, as produced bypyrolysis of naphtha or a similar charge.

Cut Cut H₂ CH₄ C₂H₂ C₂H₄ C₂H₆ C₃ C₄ C₅ ₊ CO TOTAL 15.36 28.83 0.63 29.752.92 10.18 4.94 7.355 0.035 100.00

The description is very similar to that of FIG. 1, and for this reasonit is sufficient to define the data that are significantly differentfrom the preceding.

Gas number 1, cooled in heat exchangers 2 and 3, is supplied to theseparator 4 at a temperature generally between −15 and −30° C.

Gas 5 is further cooled in the multiple-flow exchanger 6 to atemperature between −20 and −35° C. The gas circulates through a limitednumber of contact zones in countercurrent to the condensed liquid in theheat exchanger 54, which forms an integral part of the absorption column7, and it is cooled by the vaporized ethylene coming from aclosed-circuit cooling cycle. A variant not shown in FIG. 2, but whichcan be readily reconstructed by an expert in the field, consists ofpumping part of the liquid exiting the separator 11, described below, tothe top of the absorption column 7, to generate in this column a liquidproduct at countercurrent, remaining within the field of application ofthis invention.

The gas 8, collected at the head of the absorption column 7, containingonly small quantities of C₃ hydrocarbons and heavier hydrocarbons, isthen cooled in the heat exchangers 9 and 10 before being supplied to theseparator 11. The liquid product is divided into two parts: the product12 is partly vaporized in the heat exchangers 9, 6, and 2, and then issupplied to column C1 at an intermediate height. Product 13 isassociated with the cold product coming from the exchanger 20, and isheated in the heat exchangers 9 and 6 before being supplied to the topof the column C1.

The liquid product 14 coming from the absorption column 7 is also heatedin the heat exchangers 6, 2, and 16, and is supplied to column C1 inintermediate position.

The distillation column C1 is heated by a hot fluid circulating in areboiler REB1, a fluid which may be hot water from the process orlow-pressure vapor, or a combination of the two by the additional use ofa second reboiler (not shown, but familiar to an expert in the field).

The residue 18, collected at the foot of the distillation column C1, istreated so that the ethylene and other valuable products can berecovered from it, in downstream units not shown.

The gaseous product 19 coming from the separator 11 is cooled in theheat exchanger 20 by low-temperature gases and in the exchanger 21 byvaporized ethylene from a closed-circuit cooling cycle. The partlycondensed gas 22B is supplied to the separator 22 at a temperature of−90 to −100° C.

The liquid product 23 from the separator 22, which has undergonepressure reduction in gate 23A, is heated by the feed gas in the heatexchanger 20, mixes with the product 13 from the separator 11 that hasundergone pressure reduction in gate 13A.

Returning now to separator 22, the head gas 24 is cooled in theexchanger 26 to a temperature generally between −110 and −120° C. toprovide a fraction 24C which is supplied to the separator 22A. The toppart of the separator 22A receives the recycled liquid 50 after pressurereduction in gate 50A which comes from the division of the liquid pumpedfrom the separator 48 into two separate flows. The separator 48 isitself supplied by the distillate 46 from the methane rectifier 45,previously cooled to a temperature generally between −115 and −130° C.in the heat exchanger 47.

The head gas fraction 52 exiting the separator 48 is heated successivelyin the exchangers 47, 26, 20, 9, 6, 2, and 16, and is then collected.The liquid foot fraction exiting the separator 48 is pumped by a pump49. Part of the fluid from the pump 49 is sent to a pipe 51 whichincludes a gate, to the top of the separator 45.

The head gas 27 exiting the separator 22A is a mixture of hydrogen,methane, carbon monoxide and traces of ethylene. This product is heatedin the complete series of multiple-flow heat exchangers described above,and it leaves the circuit in the form of crude hydrogen. If necessary,the product 27 can be purified to obtain hydrogen at 95% in aJoule-Thompson system before leaving the limit of this part of theprocess. In addition, part of product 27 can be mixed with fraction 31prior to passage into the heater 33, to increase the hydrogenconcentration when necessary with a view to hydrogenation of theacetylene in reactor R1.

The liquid product 24A subjected to pressure decrease in gate 24B isheated in the heat exchanger 26, then is mixed with product 23 comingfrom the separator 22 after pressure decrease in gate 23A. The resultingproduct 53 is heated in the exchanger 20 by the supply gas, then ismixed with product 13 before it is heated and partly vaporized in theheat exchangers 9 and 6. The resulting product 25 is the reflux forcolumn C1.

As described above, column C1 constitutes the first remarkablecharacteristic of the invention.

The description of the treatment of product 31 from the heat distillateof column C1 is in all points similar to the description given on FIG.1, and for this reason it is not repeated here. However, it should benoted that this treatment constitutes the second notable characteristicof the invention.

The typical composition of product 31 is shown in the table below, inmol %:

H₂ CO CH₄ C₂H₄ C₂H₆ C₂H₂ C₃ ₊ TOTAL % mol. 1.42 0.015 29.12 62.14 5.991.31 0.005 100.00

The advantages over the prior art are similar to those described above.

Product 36, containing residual hydrogen, carbon monoxide, and methanein addition to ethylene and ethane, is treated in a manner similar tothe description of FIG. 1, and it is detailed in FIG. 2. Because of thegreater quantity of methane present in the head distillate 44 of columnC2, this product is treated in a methane rectification column 45, theprinciple of which is known to the expert in the field. Thus gaseousproduct 44 exiting the head of the demethanization column C2 isintroduced into the rectifier 45 after cooling and partial condensationin an exchanger 63, by vaporization of ethylene from a closed-circuitcooling cycle.

The liquid fraction collected at the foot of the rectifier 45 is pumpedby the pump 55 to provide a liquid 56. This liquid is separated into afirst flow 58, which is cooled in the exchanger 26 to provide the flow59, and in a second flow which undergoes pressure reduction in a gate57, to be mixed subsequently with the fraction 40 after cooling in theheat exchangers 6 and 41.

The treatment of the bottom product of column C1 is similar to thetreatment described in detail with FIG. 1.

The flow 59 is cooled in the heat exchanger 47, and then is separatedinto:

-   -   a first fluid 60A, passing into a gate 60, which is then heated        in the exchanger 47 to supply a fluid 62,    -   and/or a second fluid, passing into a gate 61, and which is then        mixed with fluid 60A after heating of the latter in the        exchanger 47, to provide the fluid 62.

The latter fluid 62 is then heated in the succession of heat exchangers26, 20, 9, 6, 2, and 16; in this preferred mode of embodiment, it isfinally recycled to the gas compression system outside the field ofapplication of this invention.

The elimination of acetylene from product 31 can be accomplished byabsorption and extraction with a solvent instead of hydrogenation,without deviating from the scope of this invention.

The present invention is illustrated and described according to thepreferred embodiments, but it is understood that changes andmodifications can be made by the expert in the field without deviatingfrom the field of application of the invention.

1. A process for fractionation of a substantially anhydrous gasresulting from pyrolysis of hydrocarbons, containing hydrogen andhydrocarbons, particularly hydrocarbons from C₁ to C₃, includingethylene, propylene, and acetylene, in at least one of a hydrogen and amethane-enriched current, at least one of an ethylene-enriched and anacetylene-poor current, and at least one propylene-enriched current, theprocess comprising, in stages: a) cooling and liquefying a gas resultingfrom pyrolysis of hydrocarbons, progressively, under pressure by passagethrough a series of increasingly cold successive heat-exchange zones,separating at least one condensate after passage into each heat-exchangezone, at least a first of the condensates being propylene-enriched, andat least a second of the condensates being ethylene- andethane-enriched, and containing, in solution, a smaller proportion ofhydrogen, methane, and acetylene; and collecting a residualhydrogen-rich gas; b) at least partially evaporating, by a reduction inpressure, the second, ethylene- and ethane-enriched condensate and thefirst, propylene-enriched, condensate, and heating, in at least one ofsaid successive heat-exchange zones, by thermal exchange with fluids tobe cooled, including at least the gas resulting from pyrolysis, toprovide, respectively, a first fraction at least partly evaporated dueto the reduction in pressure and the heating of the ethylene- andethane-enriched condensate, and a second fraction at least partlyevaporated due to the reduction of pressure and the heating of thepropylene-enriched condensate, to provide at least part of the coolingnecessary for the cooling and liquefying of the gas resulting fromhydrocarbon pyrolysis upon passage into the successive heat-exchangezones successively; c) introducing the at least partly evaporated firstand second fractions resulting from stage (b) into a part of adistillation column called a de-ethanizer, the ethylene- and ethane-richfirst fraction which is at least partly evaporated, at a point of saiddistillation column higher than a point of introduction of the partlyevaporated propylene-rich fraction after the partial evaporation, thepart of the distillation column operating under conditions oftemperature and pressure for separation, in an upper part, of a firsthead gas current rich in ethylene and ethane and containing, in asmaller proportion, acetylene, hydrogen, and methane, and in a lowerpart, a first bottom liquid current enriched with propylene, andcollecting the first bottom liquid current; d) sending the first headgas current from stage (c) into an acetylene elimination zoneeliminating acetylene by one of extraction with a solvent and selectivehydrogenation of acetylene with the hydrogen contained in the first headgas current to provide a current essentially free of acetylene, and e)cooling and fractionating, in a part of a distillation column called thede-methanizer, the current essentially free of acetylene to produce asecond head gas fraction enriched with one of hydrogen and methane,collecting the second head gas fraction, and a second bottom liquidfraction, enriched with ethylene and ethane, and essentially free ofacetylene, and collecting the second bottom liquid fraction.
 2. Theprocess according to claim 1, wherein the gas from hydrocarbon pyrolysisis at a pressure of 15-50 bar, and the distillation column is at apressure of 10-30 bar and, lower than the pressure of the pyrolysis gas.3. The process according to claim 1, wherein the evaporated fractionsintroduced into the de-ethanizer contain dissolved hydrogen in aproportion so that the first head gas current contains 2-10%, in moles,of hydrogen, and, in stage (d), the selective hydrogenation isessentially in ethylene with the hydrogen contained in the first headgas current of stage (c), and the temperature at the hydrogenationranges from 0° C. and 160° C.
 4. The process according to claim 1,including using only hydrogen dissolved in the evaporated fractionsintroduced into the part of the distillation column called thede-ethanizer for the hydrogenation in stage (d).
 5. The processaccording to claim 1, including obtaining at least two condensates aftersuccessive passage of the gas resulting from pyrolysis, respectively,into at least two heat-exchange zones of stage (a) and sending into anupper part of the de-ethanizer of stage (c).
 6. The process according toclaim 1, including purifying the second head gas fraction exiting thede-methanizer by distillation to recover ethylene and ethane.
 7. Theprocess according to claim 1, wherein the gas resulting from pyrolysisis a gas from pyrolysis of one of ethane and an ethane/propane mixture,and including mixing the second head gas fraction exiting thede-methanizer with the gas resulting from pyrolysis without ethylenerecovery, for treatment in mixture with the gas resulting from pyrolysisin stage (a).
 8. The process according to claim 1, including increasinghydrogen content of the first head gas current exiting the de-ethanizerby addition of hydrogen from a separator, separating a partly condensedfluid produced by refrigeration, in a heat-exchange zone, of a residualgaseous fluid flowing from the successive heat-exchange zones.
 9. Theprocess according to claim 1, including recycling part of the secondliquid bottom fraction from the de-methanizer into the de-ethanizer, toreduce acetylene concentration of the first head gas current from thede-ethanizer.
 10. The process according to claim 1, including, in stage(d), extracting acetylene with a solvent.
 11. The process according toclaim 1, wherein the first head gas current includes carbon monoxide ina concentration moderating effect on reaction catalysis in the acetyleneelimination zone.
 12. An apparatus for fractionation of a gas resultingfrom pyrolysis of hydrocarbons containing hydrogen and hydrocarbons,including ethylene, propylene, and acetylene, in at least one of ahydrogen and a methane-rich current, at least one of an ethylene-richand acetylene poor current, and at least one propylene-rich current, theapparatus including: a) means for progressively cooling and liquefyingthe gas from the pyrolysis of hydrocarbons, under pressure, by passageinto a series of increasingly colder successive heat-exchange zones,means for separating from the pyrolysis gas at least one condensateafter passage into each heat-exchange zone, a first of the condensatesbeing propylene-enriched and a second of the condensate being ethylene-and ethane-enriched and containing, in solution, a smaller proportion ofhydrogen, methane, and acetylene, and means for collecting a residualhydrogen-rich gas; b) means for evaporating, at least in part, by areduction of pressure, the second, ethylene- and ethane-enrichedcondensate and the first, propylene-enriched condensate and means forheating the condensates, independently, in at least one of thesuccessive heat-exchange zones by thermal exchange with fluids to becooled, to provide, respectively, a first fraction at least partlyevaporated, resulting from the reduction of pressure, and means forheating the ethylene- and ethane-enriched condensate, and a secondfraction at least partly evaporated, resulting from the reduction ofpressure and the heating of the propylene-enriched condensate, toprovide for progressive cooling and liquefaction of at least the gasfrom the-pyrolysis of hydrocarbons upon successive passage through thesuccessive heat-exchange zones; c) means for introducing the partiallyevaporated first and second fractions from (b) into a part of adistillation column called a de-ethanizer, the ethylene- andethane-enriched partly evaporated first fraction being admitted at apoint of the part of the distillation column higher than a point ofintroduction of the propylene-rich partly evaporated second fraction,the part of the distillation column operating under conditions oftemperature and pressure for separating, in an upper part, a firstethylene- and ethane-rich head gas current containing, in a smallerproportion, acetylene, hydrogen, and methane, and, in a lower part, apropylene-rich first bottom liquid current, which is collected; d) meansfor sending the first ethylene- and ethane-rich head gas current from(c) into an acetylene elimination zone for elimination of acetylene byone of extraction with a solvent and selective hydrogenation ofacetylene with hydrogen contained in the first head gas current, toprovide a current essentially free of acetylene, and e) means forcooling and fractionating, in a part of a distillation column called ade-methanizer, the gas current essentially free of acetylene from (d),in a second hydrogen-and/or methane-enriched head gas fraction, which iscollected, and a second bottom liquid fraction which is enriched withethylene and ethane and is essentially free of acetylene, and which isalso collected.